The exact model is an ET-500. No longer listed on either the UK or USA Yamaha sites.

It is not an air-flap governor - at least not an airflap on the flywheel and fan. I've had the cowling off and there it isn't. The usual governor lever is mounted onto a spindle that comes out of the crankcase between the motor and the dynamo. With the engine not running that spindle is easy to turn. When the engine is running the lever has a surprising amount of pull - about 6 ounces at a guess at the end furthest from the spindle.

I've had the carb totally to pieces, and I mean totally. No change. No adjusting screws (apart from a stop screw on the throttle butterfly that stops the butterfly from closing totally, and which was and remains srewed right in ie setting the highest "tickover speed" (if there was one which there isn't). With air cleaner back on and new fuel (50:1) the engine note is perfect ie just a hint of four-stroking at steady speed, and just a hint of blue haze in the exhaust, so the mixture is spot on for max power.

The governor arm has three holes in it for the spring that opens the throttle. The spring was in the middle one. I've moved it to the inner one so that the spring has less leverage on the arm - but not much change! Trouble is, to get at the spring and change it to the other hole means dismantling things again.

Loading with a 60W lamp produces a change in engine note and a slight drop in revs, but voltage gauge still showing 300 volts!

Vexed now!